test: fix or skip some failed next-gen tests

[ekexium]

What problem does this PR solve?

Issue Number: close #63343

Problem Summary:

Fix next-gen CI.

What changed and how does it work?

This PR fixes or skips some failed tests in next-gen CI

GC Worker tests (pkg/store/gcworker/):

• TestGCWithPendingTxn
• TestLeaderTick
• TestPrepareGC
• TestRunGCJob
• TestSkipGCAndOnlyResolveLock

Coprocessor tests (pkg/store/copr/):

• Fixed region boundary handling for keyspace-aware keys in NextGen mode
• Added proper keyspace encoding using store codec

Session transaction tests (pkg/sessiontxn/):

• TestFailedDMLConsistency1 and TestFailedDMLConsistency2 (require fair locking)
• Fixed TestOptimisticTxnRetryInPessimisticMode configuration

Other test fixes:

• Helper tests: proper keyspace encoding for region boundaries
• Telemetry tests: skip fair locking tests in NextGen mode
• Global system variable initialization fix for TiDBTxnAssertionLevel

All tests use kerneltype.IsNextGen() check with clear skip messages explaining the incompatibility reason.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[wuhuizuo]

/test pull-unit-test-next-gen

[tiprow]

@wuhuizuo: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test fast_test_tiprow
• /test tidb_parser_test

Use /test all to run all jobs.

In response to this:

/test pull-unit-test-next-gen

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.7274%. Comparing base (3d9a0eb) to head (570fad1).
⚠️ Report is 29 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #63355        +/-   ##
================================================
+ Coverage   72.7832%   73.7274%   +0.9442%     
================================================
  Files          1832       1833         +1     
  Lines        495794     501158      +5364     
================================================
+ Hits         360855     369491      +8636     
+ Misses       113010     109985      -3025     
+ Partials      21929      21682       -247     

Flag	Coverage Δ
integration	42.6678% <66.6666%> (?)
unit	72.8371% <66.6666%> (+0.5850%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	46.5498% <ø> (+0.0177%)	⬆️

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[wuhuizuo]

/retest

[ti-chi-bot]

@ekexium: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
non-block/pull-unit-test-next-gen	570fad1	link	false	/test pull-unit-test-next-gen

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-09-04 03:24:50.838962198 +0000 UTC m=+63834.822059646: ☑️ agreed by zyguan.
• 2025-09-05 12:25:35.608342414 +0000 UTC m=+18602.168223901: ☑️ agreed by cfzjywxk.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, yudongusa, zyguan

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Needs approval from an approver in each of these files:

• OWNERS [cfzjywxk,zyguan]
• pkg/sessionctx/variable/OWNERS [yudongusa]

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